1. Field of the Invention
This invention relates to an improved polymeric reversible optical recording medium having high dielectricity, which comprises a recording layer containing a high dielectric polymeric material and a nonionic dye, and is employable for devices, such as a photosensor, a pyroelectric sensor and a display, which utilize the pyroelectric property of high dielectric polymeric materials.
2. Discussion of Background
Recording media using high dielectric polymeric materials are widely known, and recording methods by which information is recorded in such recording media can be classified into the following three types.
The first type of the recording method is disclosed in, for example, Japanese Laid-Open Patent Applications 59-215096, 59-215097 and 61-105792. According to this method, information is recorded in a high dielectric optical recording medium by impressing an electric field to the recording medium using crossed-type electrodes, and polarizing it. The recorded information is reproduced by applying a light beam to the recorded area, and detecting whether a pyroelectric current is generated or not. The portion where the pyroelectric current is generated corresponds to the recorded bit.
The second recording method is such that information is recorded by depolarizing a once polarized portion of a high dielectric recording medium. The thus recorded information can be reproduced by applying a light beam weaker than the one used for information recording, and detecting whether a pyroelectric current is generated or not. The portion where no pyroelectric current is generated corresponds to the recorded bit; this is contrary to the first method. Such a recording method is disclosed, for instance, in Japanese Laid-Open Patent Applications 59-215096 and 59-215097.
The third recording method, reported in the "IEEE Trans. Electr. Ins." El-21, 539 (1986) and the "KOBUNSHI KAKO" 35, 418 (1986), is as follows. In general, a coercive electric field in a dielectric hysterisis curve, which curve is obtained when an alternating current field is impressed to a high dielectric material, decreases as a rise in temperature. A high dielectric recording medium is polarized because of such a property of high dielectric materials. While impressing a reverse electric field which is too weak to diverse the polarization, a light beam is applied to the polarized recording medium. The temperature of the light-beam-applied portion rises to the Curie temperature, and the polarization of this portion is diversed. Thus, information is recorded in the recording medium. The recorded information can be reproduced by applying a light beam which is weaker than the one used for information recording, and detecting the phase difference (positive and negative) in the pyroelectric current generated.
The maximum amount of the pyroelectric current generated in the first information recording method corresponds to Pt, the one generated in the second method also corresponds to Pr, and the one generated in the third method corresponds to 2Pt, where "Pt" denotes the residual polarized amount at room temperature. This means that the C/N ratio in the third method is much better than those in the first and second methods.
In order to improve the recording density in the first information recording method, it is necessary to employ finely processed electrodes; this requires higher manufacturing cost. In the second method, the recorded information cannot be partially erased. The third method is therefore considered the best one so far.
However, the most desirable structure of an optical recording medium for use in the third recording method has not yet been fixed. Recording and reproduction of information is performed by heat which is generated by a light applied to the recording medium, so that the light absorption efficiency of the recording medium is a very important factor when the third recording method is employed.
The conventional optical recording medium comprising a recording layer made of a high dielectric polymer, especially polyvinylidene (hereinafter referred to as PVD), has high light-transmission, so that it has low sensitivity. It is therefore required to use a high out-put laser for information recording. The above conventional optical recording medium is thus impractical.
Japanese Laid-Open Patent Application 63-46638 discloses a high dielectric polymeric optical recording medium comprising a dye, dispersed in a recording layer, which absorbs a light having a wavelength in the vicinity of the wavelength of a semiconductor laser beam. When information is recorded in the above recording medium, the medium is exposed to the severe conditions of high temperature and high voltage. Consequently, the dye contained in the recording medium is dissociated or decomposed after the repeated use of the recording medium, and decolored. For this reason, the recording medium of this type is also unpractical.
As described above, the high dielectric polymeric optical recording medium is exposed to the conditions of high heat and strong electric field, much severer than the conditions to which any other heat-mode optical recording media are exposed.
Since ionic dyes cannot stand such severe conditions, it is improper to employ ionic dyes in the high dielectric polymeric optical recording medium. For instance, when a cyanine dye, one of cationic dyes, is employed as a laser beam-absorbing material, it is dissociated or decomposed due to high heat and strong electric field, and decolored, resulting in a loss of the semiconductor-laser-beam absorptibity. The results of elementary analysis of the decolored dye, which was carried out by means of the secondary ion mass spectrometric analysis, are shown in FIG. 7. The chart in FIG. 7 demonstrates that chlorine ions, counter ions of the dye employed, are distributed near the electrodes with high density.
In order to utilize the pyroelectric current generated in high dielectric polymeric materials, it is necessary to impress a high electric field, irrespective of the recording methods as described above, in the course of both information recording and erasing processes. Therefore, if the recording layer contains defects or electrically chargeable impurities therein, it is easily damaged by electric discharge. It is however very difficult, from the technical and economical points of view, to prepare a recording layer which is perfectly free from such impurities or defects.
Furthermore, it is essential to employ electrodes in the recording method which utilizes the pyroelectric current generated. In general, materials for the electrode have a thermal conductivity higher than that of the materials for the recording layer. Consequently, thermal fusion at the interface between the electrode and the recording layer becomes high, and information recording with high density cannot be expected.
The conventional high dielectric polymeric optical recording media have the above-described shortcomings, so that they cannot be used practically.